Diode lasers output radiation at one frequency or frequency interval, further the diode laser may emit radiation at a further frequency or frequency interval, but that frequency, frequencies or frequency interval may not be the desired frequency interval. For example, when a diode laser is used for pumping another laser, the output of the diode laser apparatus needs to match the acceptance frequency band of the laser to be pumped. Therefore there is a need for providing a system that allows the emitted radiation to be transformed to the desired frequency or frequency interval. The term frequency and wavelength may be interchanged throughout the description using the physical relation between frequency and wavelength. Furthermore, when a laser is used for pumping another laser, the efficiency is partly determined by the beam properties of the incoming pumping beam. Therefore there is a need for optimizing the beam properties of high power diode laser based pumping lasers.
Systems and methods are described in publications such as U.S. Pat. No. 5,644,584 wherein a laser system comprising a distributed Bragg reflector or distributed feedback tunable diode laser coupled to a quasi-phasematched waveguide of optically nonlinear material is disclosed.
In the context of pumping lasers that provide ultra-short pulses, in particular Ti:sapphire lasers, the relatively high cost and complexity of pump sources such as frequency doubled diode pumped solid state (DPSS) lasers is presently limiting the applications of Ti:sapphire lasers to cost-insensitive applications. The availability of smaller and less expensive pump sources is believed to significantly expand the possible applications of the Ti:sapphire laser systems.
Recently the frequency-doubled output of high-power edge emitting diode lasers, in particular distributed Bragg reflector (DBR)-tapered diode lasers, have been suggested as an attractive source for pumping Ti:sapphire lasers. Tapered lasers comprise a ridge waveguide section coupled to an index or gain guided tapered section. The two sections may have separate electrical contacts allowing the injection of respective current drives into the different sections. Direct pumping of Ti:sapphire lasers by high power diode lasers has a number of advantages compared to other known pump sources, such as frequency doubled (DPSS) lasers and optically pumped semiconductor (OPS) lasers. Frequency-doubled DPSS and OPS lasers suitable for pumping Ti:sapphire laser are not inexpensive and up to about 50% of the cost of a Ti:sapphire laser may be attributed the pump laser. The dimensions of the frequency-doubled DPSS laser are also quite large and comparable to the dimensions of the Ti:sapphire laser being pumped. Usually the frequency doubled DPSS laser and the Ti:sapphire laser are operated as separate units and precise alignment is required. This leads to relatively high complexity of the final laser system. The complexity is further enhanced in a laser system including a Ti:sapphire laser oscillator and a Ti:sapphire laser amplifier. Here two frequency-doubled DPSS lasers precisely aligned are required.
The use of diode lasers as pump source for Ti:sapphire lasers was demonstrated in Opt. Lett. 34, 3334, 2009. Here a 1 W 452 nm GaN diode laser was used as pump source providing 19 mW of continuous wave Ti:sapphire laser power. Besides the low power efficiency, increased losses resulted from the short pump wavelength. Conventional high power diode lasers have typically been developed as broad area diode lasers with reduced beam quality in the lateral direction. This reduced beam quality will lower the overlap between the pump beam and the cavity beam of the Ti:sapphire laser and lead to relatively low efficiency. The losses induced from the short wavelength can be omitted by using longer wavelengths in the range 480-600 nm. High power diode lasers are currently not available in this wavelength range. The use of OPS lasers will lead to high conversion efficiency as the OPS laser wavelength can be tailored the absorption band of Ti:sapphire and the OPS laser has good beam quality. The complexity and price of OPS lasers are, however, similar to that of frequency-doubled DPSS lasers and will limit the applications of Ti:sapphire lasers in the same way as frequency doubled DPSS lasers.
The article “Frequency-doubled DBR-tapered diode laser for direct pumping of Ti-sapphire lasers generating sub-20 fs pulses” by André Müller et al., Optics Express, Vol. 19, 12156, 2011, has demonstrated that such a laser system can provide power levels that enable competitive direct optical pumping. The use of diode lasers as direct pump source for Ti:sapphire lasers allows the development of low-cost, ultrafast lasers with high efficiencies and small footprints. It will further be appreciated that frequency-doubled diode laser systems may be applied in a variety of alternative applications, e.g. as light source in a measuring system, display systems, medical and other diagnostic systems, etc.
For many such applications the laser apparatus should fulfil a number of criteria including high stability, high beam quality, and requirements on the generated wavelength. For example, in an ultrafast laser system, e.g. femtosecond Ti:sapphire laser system, variations in the pump power will lead to variations in the obtained spectrum and thus the pulse width. For many applications, it is of paramount importance that the pulse width is constant.
The article “Beam properties of 980-nm Tapered Lasers With Separate Contacts: Experiments and Simulations” by H. Odriozola et al., IEEE Journal of Quantum Electronics, Vol. 45, No. 1, January 2009, suggests that the beam quality of 980-nm lasers with separate current drives for the ridge waveguide and tapered sections may be improved by a stronger pumping of the ridge waveguide section with respect to the tapered section. Unfortunately, this prior art article concludes that the observed improvement, far from being a general rule, depends on the details of the device geometry. Furthermore, nothing is mentioned about the stability of the different properties of the laser.
In order to provide a laser apparatus that can be used in practical applications it would thus be strongly desirable to provide such a laser apparatus that provides a stable, high-power output of high beam quality independently of—or at least less sensitive to—the details of the device geometry.